Cordierite is a ceramic material which possesses a relatively low coefficient of thermal expansion (CTE) and a high resistance to thermal shock. Substantially pure cordierite typically has a CTE of less than 1.8.times.10.sup.-6 in./in./.degree.C. over a range of 20.degree. C.-1000.degree. C. These thermal expansion characteristics make cordierite useful in high temperature applications. For example, cordierite has commonly been used to make cookware, dinnerware, catalyst substrates, molten metal filters, mirror substrates for laser systems, and other articles which must withstand rapid changes in temperatures. Despite these favorable thermal expansion characteristics, the CTE of pure cordierite is still too high for some applications, such as mirror substrates for laser systems, catalytic substrates, molten metal filters and the like. For example, most automobile manufacturers require automotive catalytic converter substrates to have a CTE of less than 1.25.times.10.sup.-6 in./in./.degree.C.
Cordierite compositions having a lower CTE have been developed. For example, anisotropic cordierite compositions made from raw platelet type clay have a reduced CTE which allows them to meet automobile manufacturers' standards of CTE for catalytic converter substrates. However, these cordierite compositions have low and inconsistent porosity which causes difficulties in the application of catalytic coatings to catalytic substrates. In addition, cordierite compositions containing germanium dioxide have a greatly reduced CTE which is substantially zero between room temperature and a selected second temperature up to 400.degree. C. However, these cordierite compositions are not suitable for use as catalytic substrates because of the high cost of germanium dioxide at the percentages contained in these compositions.
Anisotropic cordierite compositions are disclosed in U.S. Pat. No. 3,885,977 (hereinafter "the '977 patent"). According to the '977 patent, cordierite compositions having a CTE which is reduced and different directionally in the composition can be produced by selecting raw clays which are in platelet form or which can be broken down into platelet form, mixing the clay with other appropriate raw materials, properly orienting the clay platelets in the mixture, and firing the mixture to form cordierite. These cordierite compositions are described as having a CTE in the axial direction of as low as 0.55.times.10.sup.-6 in./in./.degree.C. and 1.1.times.10.sup.-6 in other directions which is less than that required by most automobile manufactures. However, as stated hereinabove, the cordierite compositions produced according to the '977 patent are relatively low in porosity. Catalytic substrates are typically coated with a precious metal catalyst coating. This coating normally consists of a gamma alumina wash coat followed by the application of a coating of precious metal such as platinum. It is more difficult to apply such a coating to less porous cordierite compositions than to more porous cordierite compositions.
U.S. Pat. No. 4,403,017 (hereinafter "the '017 patent") discloses germanium dioxide modified, high-density, cordierite compositions which are useful to make mirror substrates for laser systems. According to the '017 patent, cordierite has an essentially zero CTE between room temperature and a second selected temperature when a sufficient amount of germanium dioxide is incorporated into the cordierite structure. The temperature at which the CTE of the cordierite becomes zero depends on the concentration of the germanium dioxide in the cordierite. In addition, increasing the concentration of the germanium dioxide in the cordierite increases the temperature at which the CTE of the cordierite becomes zero and reduces the average CTE of the cordierite. The '017 patent teaches that the proper proportion of germanium dioxide in cordierite is a 0.2 to 2 mole substitution of germanium dioxide for silica in the cordierite. This corresponds to a germanium dioxide concentration of about 3.6 to about 27 weight percent in the cordierite. According to the '017 patent, cordierite having a germanium dioxide concentration of greater than 3.6 weight percent has a thermal expansion lower than that of pure cordierite at temperatures greater than 124.degree. C.; however, cordierite having a germanium dioxide concentration of about 3.6 weight percent does not exhibit reduced thermal expansion at temperatures of 120.degree. C. or greater. Instead, the '017 patent shows that the thermal expansion of cordierite with a germanium dioxide concentration of 3.6 weight percent is higher than that of pure cordierite at the same temperatures.
U.S. Pat. No. 4,587,067 (hereinafter "the '067 patent") discloses a method for the production of germanium dioxide modified, high-density, cordierite compositions for use in making mirror substrates for laser systems. According to the '067 patent, the CTE of cordierite can be made substantially zero over selected broad temperature ranges up to 200.degree. C. by substituting 0.1 to 4 moles of germanium dioxide for silica in the cordierite structure. This corresponds to a germanium dioxide concentration of from about 1.8 to about 43 percent by weight in the cordierite. The '067 patent does not show that a concentration of less than 3.6 weight percent germanium dioxide in the cordierite is effective to reduce the thermal expansion of the cordierite at the elevated temperatures. Thus, the prior art teaches that the presence of more than 3.6 weight percent germanium dioxide in cordierite is necessary for the reduction of the thermal expansion of cordierite at temperatures greater than about 124.degree. C.
As provided hereinabove, the CTE of pure cordierite is too high for certain applications. According to the prior art teachings, germanium dioxide must be present in the cordierite in the amount of at least 3.6 weight percent to be effective in reducing the thermal expansion of the cordierite at temperatures greater than about 124.degree. C. Automotive catalytic converter substrates are commonly subjected to temperatures greater than 124.degree. C. Thus, based on the prior art, cordierite with a germanium dioxide concentration of 3.6 weight percent or less would have a CTE too high to meet standards set by automobile manufacturers. Because of the high cost of germanium dioxide, cordierite having more than 3.6 weight percent germanium dioxide is not economical to produce.
There are additional problems in the production of catalytic substrate cordierite compositions. One such problem is that the mixture of raw materials used to make cordierite tends to shrink during drying and firing. If the shrinkage occurs too rapidly, the forming cordierite becomes cracked or distorted. Accordingly, the drying and firing steps of the standard cordierite production process must be conducted slowly. This time consuming process is costly to conduct.
Therefore, there is a need for a cordierite composition having a reduced CTE and a relatively high level of porosity, which can be produced economically. More specifically, there is a need for an economically-produced cordierite composition having a reduced CTE and improved coating characteristics which is useful as a catalytic substrate.